1. Field of the Invention
The present invention generally relates to a cable equalizer and, more particularly, to a multi-stage cable equalizer.
2. Description of the Prior Art
With the arrival of the information era, the Internet has become the most important media that carries information of video, audio, data and so forth. Therefore, the attenuation loss of the transmission cable becomes an important issue in the industry.
When information data is transmitted on a cable at high frequencies, the equivalent impedance of the cable results in attenuation loss in response to the transmission distance. In other words, the attenuation loss increases as the transmission distance and the frequency increase.
Please refer to FIG. 1, which is a diagram showing the frequency response of a transmission cable depending on the cable length. In FIG. 1, different lengths of cables result in different attenuation on the transmission signal. More particularly, larger attenuation in gain appears for a long cable at high frequencies.
In order to compensate the loss with the increase of distance at high frequencies, it is usually that an equalizer is placed at the receiving end of a transmission system to reconstruct the transmitted signal over the cable.
FIGS. 2A, 2B and 2C show the frequency response of the transmission cable, the frequency response of an equalizer and the frequency response of the transmitted signal over the cable after passing through the equalizer, respectively. In FIG. 2A, different lengths of cables result in different attenuation on the transmitted signal. Therefore, an equalizer for long cables and an equalizer for short cables with frequency response as shown in FIG. 2B can be used to compensate the loss in the long cable and the short cable, respectively. FIG. 2C shows that the attenuation loss is compensated for both the long and the short cables by using the equalizers for long cables and short cables. Therefore, the transmitted signal can be constructed using the appropriate equalizers for both the long and the short cables.
In general, cable equalizers can be divided into the fixed gain equalizer and the adjustable gain equalizer. The fixed gain equalizer only compensates the loss in response to a fixed frequency range, for example, the frequency range higher than a fixed frequency. And then, the adjustable gain equalizer compensates the loss in response to different frequencies. Therefore, the fixed gain equalizer has less flexibility than the adjustable gain equalizer. However, the former costs less and is easier to fabricate, hence exhibiting higher reliability.
Therefore, there is need in providing a multi-stage cable equalizer so as to compensate the loss caused by the transmission cable in response to its length while achieving better compensation and reduced cost.